The announcement was met by applause at an event in Cupertino.
"We weren't the first to this party, but we're going to be the best," Jobs said Thursday.
The iPhone already permits some multitasking, but that's largely limited to Apple's own programs. Apple had not given users ways to seamlessly switch among all the software "apps" available from outside software companies, the way phones from rivals Palm Inc. and Google Inc. already do.
Shares of Apple fell 27 cents to $240.33 in afternoon trading Thursday.
The AP contributed to this report.